Telegram for iOS to be replaced with new app based on Apple's Swift language
Since then, the developer has been tweaking Telegram X based on users feedback, in order to make it ready for prime time. Durov confirmed that Telegram X is now ready to replace the current Telegram app for iOS, something that will happen within the next week or two.
Telegram X is based on Apple's Swift language, which is why it's faster and more efficient than the standard version of the app. However, because Telegram X relies on an entirely new codebase, the app may have some minor bugs and glitches, but the folks at Telegram promise to fix them as quickly as possible.
source: Telegram via 9to5mac